The present invention relates to electrophotographic imaging, but more specifically to a method and an apparatus that compensates for variations in toner concentration based on machine characteristics.
In electrophotographic imaging, also known as xe2x80x9cxerography,xe2x80x9d a charge retentive surface, typically known as a photoreceptor, is electrostatically charged and subsequently exposed to a light pattern of an original image to selectively discharge corresponding image areas on the charge retentive surface. The resulting pattern of charged and discharged areas on the photoreceptor forms an electrostatic charge pattern, known as a latent image, conforming to the original image. The latent image is developed by contacting it with a developer material that includes a finely divided electrostatically sensitive powder, i.e., toner particles. Toner is deposited and held on the image areas by an electrostatic charge of the photoreceptor. Thus, a toner image is produced on the photoreceptor in conformity with a projected image of an original document being reproduced. The toner image is then transferred to a substrate, such as a copy sheet of paper, whereupon the image affixed to the substrate is subsequently fused to form a reproduction.
The step in the electrophotographic process in which the toner is applied to the latent image on the photoreceptor is known as xe2x80x9cdevelopment.xe2x80x9d During development, varying amounts of toner are placed on the latent image so that the toner particles adhere to charged regions on the surface of the photoreceptor in conformity with the latent image. The amount of toner applied during the development stage determines the image density and color of the corresponding latent image and the reproduction.
Many development techniques are known in the art and. a number of them require uniformly mixed toner particles in a mixture of xe2x80x9ccarrierxe2x80x9d and toner particles. Generally, toner plus carrier equals xe2x80x9cdeveloper,xe2x80x9d also known as a two-component developer. Toner particles are extremely fine and variably responsive to electrostatic fields. Carrier particles comprising ferromagnetic beads, for example, are relatively larger and respond to magnetic fields. In a xe2x80x9cmagnetic brushxe2x80x9d development system of the type disclosed by U.S. Pat. No. 5,946,534 to Lewis and assigned to the same assignee hereof and U.S. Pat. No. 4,690,096 to Hacknauer et al., a development unit, typically one for each separated color, carries a two-component developer mixture in a sump or reservoir that contains augers, transport rolls, and developer rolls. Augers admix the toner and carrier particles, development rolls apply toner to the latent image, and transport rolls or other mechanisms transport mixed particles from the sump to the development rolls.
Contact between toner and carrier during the mixing process triboelectrically charges the toner particles of the mixture causing them to electrostatically adhere to the carrier particles. The developer roll provides a relatively strong rotating magnetic field causing the carrier particles of the mixture to form brush-like strands, much in the manner as iron filings when exposed to a magnetic field. What is thus formed is a brush of magnetic particles with toner particles adhering to the strands of the xe2x80x9cmagneticxe2x80x9d brush. During the development process, this magnetic brush is swept across the latent image of the photoreceptor so that toner particles separate from the carrier particles and adhere to the photoreceptor in accordance with the magnitude of latent image charge residing on the photoreceptor. The xe2x80x9cdevelopedxe2x80x9d latent image then moves to the next processing station in the imaging machine while the carrier particles are stripped from the developer roll and re-circulated to the sump for subsequent reuse.
During a reproduction job, the amount of toner consumed at the development roll to develop the latent images depends, in substantial part, on the coverage area in the original document being reproduced. Toner demand will often vary print-to-print subjecting the developer unit to varying degrees of toner consumption. To achieve acceptable toner density in a reproduced image, i.e., image quality, toner must be replenished on an on-going basis so as to maintain sufficient toner concentration in the developer mixture. Development units containing a relatively large mass of developer are not as susceptible to toner concentration fluctuations when image coverage areas fluctuate during a reproduction job stream. Due to mixing/transport delays, as well as delays resulting from toner replenishment after depletion, small mass developer housings are more sensitive to such fluctuations.
The desire to combine multiple development units in a compact imaging system necessitates relatively small developer housings for each of the respective colors. Small housings having a low developer mass suffer from rapid fluctuations in toner concentration (TC) as the image area coverage varies from print-to-print. Toner concentration also fluctuates due to delays in mixing and transport of the developer material. Mixing and transport delays are typically about 10 to 30 seconds due to the time for required for toner replenishment, for admixing to occur, and for transporting the developer to the latent image development rolls. If the coverage area of a print job stream is known sufficiently far in advance, toner can be dispensed and developer can be admixed so that a mixture of the proper toner concentration arrives at the development rolls at the instant the latent image arrives at the development station.
When pixel counting in the original is used to determine coverage area, the required toner concentration information is typically known only a few seconds in advance of development, which is often too late to provide effective toner concentration control. The time required for transport and mixing, on one hand, and the relatively shorter pixel count lead time, on the other hand, cause unwanted fluctuations in toner concentration, which is exacerbated in small mass developer housings. In high quality color reproductions, such variations produce objectionable print quality variations and other deficiencies.
In the course of machine usage, toner concentration levels may also xe2x80x9cdriftxe2x80x9d or vary significantly with aging of the machine. Numerous techniques have been devised to address such long-term variations. One technique includes periodic monitoring of test images using a density sensor at the development station, and then adjusting the field strength and/or other parameters (charge, exposure, developer bias, halftone threshold, or a combination thereof) at the developer unit in order to compensate for deviations. While this may be viable to compensate for long-term drift (e.g., over several hundreds or thousands of reproductions), it is not always adequate to compensate for short-term fluctuations in toner concentration. When image coverage area rapidly fluctuates from print-to-print, a delay of a few seconds between development of a test patch and sensing of density prevents complete compensation of toner concentration fluctuation. The delay also makes it difficult to effect control of electrostatic set points, exposure time, and/or toner concentration to attain high quality reproduction. In addition, the need for print-by-print density information required to make short-term adjustments requires excessively high bandwidth on the data bus between the sensor and the printer""s central control unit. Such high data bandwidth negatively impacts other automated functions of the imaging machine that require bandwidth usage, and unnecessarily requires complex test patch generation and detection algorithms. Also, the need for rapid response with this latter approach makes the system susceptible to noise.
The present invention addresses the aforementioned and other problems associated with short-term and long-term fluctuations in toner concentration in an electrophotographic or electrostatographic imaging system.
In accordance with one embodiment of the invention, there is provided a method of dynamically controlling image density in an electrophotographic imaging system that includes at least one development station for applying toner to an electrostatic latent image comprising generating a software behavioral model of toner concentration, toner mixing delay, and/or transport of toner at various stages during a development cycle of the latent image and dynamically adjusting the amount of toner applied to the latent image according to the software behavior model during a reproduction job stream. The method may also include altering electrostatic set points of an electrostatic controller of the imaging machine to maintain a desired toner density in the latent image; controlling toner concentration in a two-component developer mixture of toner and carrier particles; and/or correcting the software behavior model according to actual measurements of behavior of at least one of toner concentration, mixing delay, and transport delay during a reproduction job stream.
The method is particularly suited but not limited to controlling short-term fluctuations of toner concentration in small mass developers. In addition, the method may be supplemented to correct long-term fluctuations and/or drift using a conventional density sensor to less frequently monitor toner density.
In accordance with another aspect of the present invention, there is provided a toner density control method that uses software modeling of an imaging machine to compensate for long-term and/or short-term variations in toner concentration at a developer roll of the machine. A model of developer dynamics is maintained so that a central controller of the imaging machine may approximate in advance required set points or other parameters of the developer to compensate for toner deficiency based on a projected time delay for a replenished supply of a toner-depleted developer mixture to reach the development roll, which time delay may include a delay introduced during mixing or transport of toner and carrier particles from the dispenser to the developer rolls.
In accordance with another aspect of the invention, an apparatus is provided that dynamically controls image density in an electrostatographic imaging system having at least one development station for applying toner to an electrostatic latent image where the apparatus includes a software model that emulates the behavior of at least one of toner concentration, mixing delay of toner, and transport of toner at various stages during a development cycle of the latent image; and a controller that dynamically adjusts the amount of toner particles applied the latent image according to the software model during a reproduction job stream.
In yet another aspect of the invention, a controller adjusts the approximated time delays of a model in accordance with actual measurements of short-term variations.